Forbidden and Dangerous
by batman-defeats-all
Summary: After years of being defeated by the successful superheroes the Tele-Twins, criminal genius Gideon Gleeful hires the infamous super villain Nightmare to dispose of his only roadblock to conquest. No one expected sparks to fly when Nightmare faced Lady Thoughts- the question is just if the sparks will start a fire that could destroy everything they know. Mabill, Superhero!AU
1. Prologue

**I do not own Gravity Falls.**

It was growing dark out; the sun was slowly setting over the city skyline, painting the glass panes on the towering building various hues of orange. The sounds of the a bustling town still roared on through the approaching night, cutting through the chilling air with every yell and honk.

High above the city, however, there was silence. Behind thick windows, lit only by hanging lamps on the ceiling in the dwindling sunlight, the quiet was still prominent within the walls of the private office.

At least, it did before the man whose name resided on the door of the office, not to mention the large lettering on the top of the building, slammed the metal door open, the contact with the wall ringing through the room. Closing the door with equal force, he stormed to the desk near the window, seething, removing his powder blue suit jacket as he walked.

"Damn those twins!" Gideon Gleeful growled to himself, collapsing in his desk's leather chair with a frustrated huff. "Can I not go a single day without those super-freaks deciding to stick their noses in my business?"

Breathing heavily, his bright blue eyes closed and his hands running through his heavily styled white hair, he slowly picked up a newspaper laying face down on his desk.

"Seeing an article about my newest business deal would be a good cheer-up, in the least," he mused, straightening the paper.

As his eyes met the text, however, they widened immediately.

"Are you freaking kidding me?" he hissed, throwing the newspaper away from himself. It fluttered through the air, some pages separating from the rest as it descended, before the front page story landed face up, the heroes in the large picture smiling blindly at the lamps above.

"Tele-Twins Find Cure for Deadly Sickness" the headline read, the heroes themselves standing side by side, their brown curls touching due to proximity, each supporting a grinning child. From his seat, Gideon glared over the stacks of paper on his desk at the single sheet.

"Their only power is being telepathic, and just with each other," he whispered weakly his fingers once again in his hair, strands of white falling over his forehead. "How the hell does that help them figure out the cure to a scientifically-engineered sickness? They can think in each other's mind. That doesn't tell you how to ruin literally any plan I concoct!" Gideon slammed his palms onto his desk as he finished, his eyes wild.

"Mr. Gleeful?" his head turned to the doorway swiftly as his name was spoken, his eyes softening almost instantly when he saw his secretary and old friend Pacifica Northwest standing in the doorway, a clipboard clenched tightly in his delicate hands.

"Umm, yes?" he said quickly, trying to make himself look more dignified as he pushed his stray hairs back from his forehead.

"Your five o' clock appointment is here."

Gideon blinked in confusion.

"I... don't remember scheduling one..."

Pacifica sighed.

"Gideon, it's not exactly an _official business _meeting."

Gideon's eyes widened in memory.

"Ack, of course. I had forgotten. Send him in, Pacifica."

"Of course sir."

As she left, Gideon quickly struggled to put back on his suit jacket, and was slicking his hair back with hair gel when the lights suddenly went out.

"Wha-" he started to exclaim, only to be interrupted when he saw a sudden glow inches from his face. "Ahhh!" he fell back into his chair.

Gideon scowled when the lights came back on, the sound of a nasally laughter ringing out.

"I assume you're the infamous 'Nightmare' I've heard so much about?" Gideon asked in an annoyed voice as he saw a hooded figure in the middle of his office.

The figure pushed down his hood, revealing his shaggy brown hair, completely covering one of his eyes, which seemed to be glowing faintly in the half-light, bright yellow uncovered eye, deep bags underneath said eye, and a sinister smirk.

"Bill Cipher, the thing of Nightmares, at your service, Mr. Gleeful!" he replied, still grinning, as he dramatically bowed, sweeping his cape behind himself.

"Was the flashy entrance really necessary?" Gideon sighed, rubbing his forehead with one hand.

"Yes."

Gideon appeared like he wished to reply to the statement, but eventually just shook his head, dropping the topic.

"Does your presence mean that you're willing to assist me?" Gideon droned, folding his hands on top of his desk.

"It depends on what you're asking."

Gideon smirked back at Bill.

"Are you familiar with the Tele-Twins?" he asked in a mock casual voice.

He felt victory within his grasp as the glow behind the tufts of hair suddenly gave a bright flash.

"When do I start?"

Gideon laughed, leaning back. Soon his plans would have nothing keeping them from being fulfilled.


	2. Chapter 1

**I do not own Gravity Falls.**

Mabel Pines slumped into the apartment she and her brother shared, a groan escaping her lips as she kicked the door closed. With a huff, she collapsed on a plush couch only feet away from the door, letting his form slowly sink in the cushions. Closing her eyes, she breathed out of her nose.

"Dippppeeer!" she moaned loudly, her voice carrying through the apartment. The corner of her lips turned up when she heard loud crashing come from a different room. Moments later, a young man with a head full of wild brown curls feel out of the doorway to the room, coughing, billows of smoke following him.

"Mabel, I was busy-" he said between coughs as he quickly turned off the fire alarm.

"You don't have to work at Gleeful Industries," she hissed, interrupting him. "You owe me attention."

Dipper rolled his eyes as he wiped a thin layer of ash from his forehead, before sitting next to his twin.

"You know we have to keep an eye on Gideon-"

"_He flirts with me, your argument is invalid," _her voice was in a hiss once again, though Dipper could fully understand her frustration. Having to not only work for the one who caused so much destruction, but to have him show... _ interest _in her must be maddening.

He wished she didn't have to have that job, but they did need to keep an eye on Gleeful. He was dangerous; against his better judgment, Dipper would even have to admit that Gideon could be rather impressive. Despite his age of 21, he already had a major corporation that he was in complete control of, and had been for multiple years now.

Yet, he used that power for all the wrong things. And so Dipper and his sister had to continuously defeat the white-haired brat, and due to that Mabel had a job she despised.

"I was trying to figure out how he got that sickness to spread so quickly," Dipper muttered underneath his breath, crossing his arms and looking away from his sister.

"Aww, come on, you can totally do that after patrol, right?" she asked hopefully, giving him a playful shove as she spoke.

Dipper glanced at Mabel, before pinching the bridge of his nose in frustration.

"_Fine,_" he breathed out. Looking up, he leaned back into the couch. "Any news from work?"

"He bought out McGucket Robots," Mabel answered glumly.

"So now he has access to large, powerful machinery at a moment's notice," Dipper covered his face with both hands. "Any other good news?" he continued, his voice dripping with frustrated sarcasm.

Mabel furrowed her eyebrows.

"There was... this really weird guy heading to Gideon's office as I was leaving..."

Dipper sat up, looking sharply to her.

"... What do you mean?"

"He had this big cape, which was held up on his chest with this strange triangle symbol, and he had this really dark hood up. He bumped into me as I was heading out, and while I was apologizing all I could see from under his hood was... this _smirk."_

"... Well, that doesn't bode well..."

Mabel nodded, before standing suddenly, her hands on her hips.

"Well, Mister Mind, are you ready to get into costume?"

Dipper smiled, also standing.

"Always, Lady Thoughts."

Fifteen minutes later, they were both clutching grappling hooks, their gloved fingers digging into their palms as they swung from rooftop to rooftop.

'Quiet night,' Dipper heard his sister's voice speak out in his mind.

'Whenever you say that, something big happens...'

'Ahhh, but my dear brother, I didn't say it! I _thought _it!'

Dipper looked to Mabel with a frustrated expression.

Eventually, they stopped moving, settling on the roof of the Gleeful Industries tower, both looking up at the lettering with a scowl.

Eventually, Mabel broke her heated gaze.

'He's probably still licking his wounds after last time, we should move on,' she thought to Dipper, disgust lacing her tone.

Dipper's scowl remained for a moment before he nodded.

'… Yeah,'

As the twins jumped to a lower building, they didn't notice the shadow that followed their movement.

They continued patrolling, breaking up a small fight or robbery here and there, but overall being rather inactive. After another few hours, Mabel sighed.

'It seems like anything that was gonna happen tonight has, so we should probably start heading ba-'

She cut off her thought when she saw a sudden shine of light directly behind her brother.

She didn't know why it made her stop. It could just be a ray of moonlight, or a reflection from one of the buildings.

Yet, as soon as she saw it, a great sense of danger swept over her.

She felt that justified when a blade was suddenly visible behind Dipper.

"Mister Mind, look out!" she screamed, much to Dipper's surprise. He turned his head, instantly ducking when he saw the blade. He rolled away from the unseen enemy swiftly, returning to a crouch next to Mabel.

"You're more observant than you look," a high, nasally voice rang out as the figure that had attempted to attack Dipper stepped into the dim moonlight, causing Mabel to suddenly gasp.

'That's the guy I saw at work!' she called out to her brother through her mind.

The only other voice the two of them had ever heard within their mind zone was each other. It was the place they could be alone, they could be private. It was their safe place.

Which was why they both felt their blood chill as they heard the voice of another.

'Oh, you're working for the enemy! That's clever!' his voice was mocking, brimming with a hidden humor, as if he was telling some sort of joke.

Mabel doubted it was a joke she would find funny.

"H-how?" she eventually managed to choke out, staring intently at the figure in slight horror.

There was a burst of wind, a sudden chill, and without her even realizing it the man was inches from her face, his single eye not covered by hair gazing into her own, bony fingers gripping her shoulders.

Her breath stopped when he leaned closer, his mouth stopping directly next to her ear.

"You're not the only one with powers, beautiful," he whispered softly, his hot breath playing on her cheek.

Mabel wanted to move at him; her body screamed at her to throw a punch, to kick him away, to fight back.

Yet, for some reason, she was paralyzed, her eyes fixed on the longs strands of hair that covered what should have been an eye.

All there was, though, was a dull glow. A distracting... beautiful dull glow...

"Get away from her!" Dipper's voice cut through her trance, shocking her back to reality.

She took a step back as her brother threw himself at the hooded figure, a fierce snarl painted across his face.

"Overprotective, I see," the man chuckled darkly.

"When creeps like you bother her, why shouldn't I be?" Dipper growled in response, his fist raised.

"Well, I can't argue with that," his lips dipped upward in a condescending smile. "This has been a quite an educational mission."

Mabel's masked eyes narrowed.

"What does that mean?"

He smirked again, then vanished.

"Wha-" Dipper started to speak, only to fall to the rough concrete moments later. Brushing himself off and straightening his cape, he looked back at Mabel.

"He didn't hurt you, right?"

She shook her head.

Dipper smiled in relief.

"Let's get home, then, before anything else crazy happens."

Elsewhere in the city, Gideon looked up from his paperwork at the sudden rush of wind.

"You just left a few hours ago, surely you aren't finished yet," he commented softly.

"Well, of course not," Bill spat. "I just wanted to inform you that the first seeds have been planted."

Gideon raised a eyebrow in interest.

"How?"

Bill grin caused even Gideon to feel immense discomfort shoot instantly through his body.

"I'm called Nightmare for a reason, Mr. Gleeful. And once I've burrowed into someone's head, it's hard to get me out."


End file.
